An insulating pipe joint is widely used as a component indispensable to transportation of gases and liquids such as a coolant Freon gas. Most of the conventional insulating pipe joints are small structural elements, and large insulating pipe joints having satisfactory airtightness and watertightness are not commercially available. The present invention generally concerns an insulating pipe joint having improved water- or air-tightness.
With the recent increase in the oil price, active efforts are being made to recover oil from sand oil reserved in Canada and Venezuela. The oil sand deposits are found about 500 m below the ground and form a layer about 50 m thick. The oil in the oil sand is very viscous and cannot be recovered at ordinary temperatures by conventional pumping. Instead, steam is injected into the oil sand formation and the temperature of the oil is increased to reduce its viscosity to a level that can be lifted by a pump. But to achieve more efficient and economical recovery, intensive studies are being made on the method of using two steel oil recovery pipes each having an electrode on the end stuck in the oil sand deposit; in this method, the two pipes are positioned about 30 to 100 m apart, and a voltage of about several hundred to thousand volts is applied between the electrodes and the Joule's heat produced increases the temperature of the oil sand deposit to thereby reduce the oil viscosity. Since the oil sand deposit has a specific electrical resistance several times as high as that of the overlying stratum, an insulating pipe joint must be provided between the steel pipe in the stratum and each of the electrodes buried in the oil sand layer. Without an insulating pipe joint, an electric current flows through the stratum above the sand oil layer, not between the electrodes. This is the primary reason for the recent rapid increase in the demand for the insulating pipe joint.
The insulating pipe joint that can be used for the above purpose must meet several requirements. First of all, it must withstand a voltage as high as several hundred to thousand volts. Secondly, it must have the desired airtightness or watertightness (capable of withstanding a water pressure of about 85 kg/cm.sup.2 multiplied by the safety factor) not only at ordinary temperatures but also at the temperature (about 300.degree. C.) that can reduce the viscosity of the oil sand layer. Thirdly, the joint must have a mechanical strength sufficient to support the electrodes, or a mechanical impact strength that protects the joint from breaking upon contact with the wall of a hole made in the oil sand layer to receive each electrode.
An insulating pipe joint meeting these requirements is basically made of two electrically conducting pipes connected by an insulator that also provides air- or water-tightness. Of various factors that govern the stated requirements for the pipe joint, the most important is the insulator. The requirements are also closely related to the metallic material of which the pipe is made, as well as the construction of the joint, but these factors are largely governed by the insulator.